Clean verif/ directory before writing to it

[Lysxia]

When you remove a function in your Rust code, the corresponding Coma file(s) should be removed.

[arnaudgolfouse]

Hm, this is not enough unfortunately: removed files are still listed in the why3session.xml, and they cause a crash in why3...
Granted, why3find handles it without issues ;) (But I need the ide and its tactics)

[Lysxia]

I think we want to fix why3 here.

[Lysxia]

This should resolve the crash https://gitlab.inria.fr/why3/why3/-/merge_requests/1189

[jhjourdan]

This should resolve the crash https://gitlab.inria.fr/why3/why3/-/merge_requests/1189

Is this really what we want? Who is going to remove the session file in this case?

[Lysxia]

My Why3 MR addresses the current/old use case of having one big why3session.xml at the root of the project for all .coma files, by fixing the crash when a file is removed and instead treating its session as detached.

Since then, we've also decided to switch to a workflow where each .coma file has its own why3session.xml or proof.json, and this workflow does not run into the crash above. As far as I can tell, dangling why3session.xml files are harmless. Removing them as soon as we remove their .coma file seems dangerous (the user could change their mind and re-add the .coma file, or they may want to move the session/proof file rather than delete it). Instead I propose to add a new command cargo creusot clean to remove dangling sessions and proofs. cargo creusot could suggest this command in a warning.

[jhjourdan]

Sounds good indeed!

[jhjourdan]

Ok, this looks good to me. Merging as soon as CI passes.